Record PendingTaskCount when a backgrounded renderer is suspended.

third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/scheduler/renderer/renderer_scheduler_impl.h"
 
 #include "base/bind.h"
 #include "base/debug/stack_trace.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 472 matching lines @@
   suspend_timers_when_backgrounded_closure_.Cancel();
   ResumeTimerQueueWhenForegrounded();
 }
 
 void RendererSchedulerImpl::SuspendRenderer() {
   helper_.CheckOnValidThread();
   DCHECK(MainThreadOnly().renderer_backgrounded);
   if (helper_.IsShutdown())
     return;
   suspend_timers_when_backgrounded_closure_.Cancel();
+
+  UMA_HISTOGRAM_COUNTS("PurgeAndSuspend.PendingTaskCount",
+                       GetNumberOfPendingTasks());
+
   // TODO(hajimehoshi): We might need to suspend not only timer queue but also
   // e.g. loading tasks or postMessage.
   MainThreadOnly().renderer_suspended = true;
   SuspendTimerQueueWhenBackgrounded();
 }
 
+int RendererSchedulerImpl::GetNumberOfPendingTasks() {

[Sami, 2016/10/14 01:38:18]

I would suggest adding this getter into TaskQueueManager instead so it can just
iterate over |queues_| there. Otherwise it is easy to forget to add a queue here
when we introduce new ones.

[tasak, 2016/10/14 05:55:32]

I see. I agree that TaskQueueManager is the best class to add the getter.
Done.

+  int count = 0;
+  for (const scoped_refptr<TaskQueue>& loading_queue : loading_task_runners_)
+    count += loading_queue->GetNumberOfPendingTasks();
+  for (const scoped_refptr<TaskQueue>& timer_queue : timer_task_runners_)
+    count += timer_queue->GetNumberOfPendingTasks();
+  count += DefaultTaskRunner()->GetNumberOfPendingTasks();
+  count += CompositorTaskRunner()->GetNumberOfPendingTasks();
+  count += idle_helper_.GetNumberOfPendingIdleTasks();
+  return count;
+}
+
 void RendererSchedulerImpl::EndIdlePeriod() {
   if (MainThreadOnly().in_idle_period_for_testing)
     return;
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("renderer.scheduler"),
                "RendererSchedulerImpl::EndIdlePeriod");
   helper_.CheckOnValidThread();
   idle_helper_.EndIdlePeriod();
 }
 
 void RendererSchedulerImpl::EndIdlePeriodForTesting(
@@ skipping 1077 matching lines @@
     case v8::PERFORMANCE_LOAD:
       return "load";
     default:
       NOTREACHED();
       return nullptr;
   }
 }
 
 }  // namespace scheduler
 }  // namespace blink